Due to the increasing attention to terrorist activities, there has been increased interest in providing more effective and more efficient systems for inspecting cargo at points of entry and to identify contraband, particularly explosive and fissionable material. While the smuggling of contraband onto planes in carry-on bags and in luggage has been a well-known, on-going concern, a less publicized but also serious threat is the smuggling of contraband across borders and by boat in large cargo containers.
The development of systems for large container content control has gone in two different directions.
1. The first direction is a follow-on to X-ray machines, with high-energy (2.5 to 9 MeV) radio frequency (RF) electron linear accelerators (linac) generating bremsstrahlung radiation.
In an RF linac, an electromagnetic wave is used to accelerate charged particles. There are two types of RF linac: traveling wave and standing wave. The traveling wave linac is a circular waveguide with diaphragms which slow the speed of the wave down to the speed of particles being accelerated. The speed of electrons with energy above 0.5 MeV is about speed of light. The standing wave linac is a chain of coupled cavities with the length of each close to half the wavelength of electromagnetic wave. Most electron RF linacs operate at a wavelength of 10 to 10.5 cm (i.e., a frequency of 2998 to 2856 MHz), and this wavelength band is named S-band. To accelerate the electron beam to 10 MeV in a traveling wave linac, its length must be 2.2 to 2.5 m, and it is necessary to install a solenoid above the waveguide for particle focusing. The standing wave linac for the same beam energy is about two times shorter and does not require the focusing solenoid; however, the RF source must be protected from reflected wave by the high power circulator. In both types of linacs, to produce an accelerating field, 2.5 to 3 MW of pulsed RF power must be spent, and about 1 to 1.5 MW RF power will be transferred to the beam, so the total RF power necessary for a cargo inspection linac is 3.5 to 4.5 MW. By decreasing the length of the electromagnetic wave, e.g., going to C-band (5.5 to 25 cm), the linac length and the RF power required to produce an accelerating field are decreased, approximately 2 and 1.5 times respectively.
The RF linac generates bremsstrahlung radiation. Bremsstrahlung (or braking radiation) is produced when electrons hit the so-called bremsstrahlung target. To generate the maximum number of photons, the target is made of a heavy element material with high melting temperature, e.g., tungsten or tantalum, with a thickness 1.5 to 2 mm. At 10 MeV, 8 to 10% of the electron energy is transformed to the energy of the X-ray radiation. The energy spectrum of the generated X-ray radiation is continuous, with the end-point energy equal to the electron energy and the number of photons increasing with the decrease in energy. The X-ray energy spectrum can be hardened using so-called energy filters—a light element absorber installed after the bremsstrahlung target.
A 2.5 to 9 MeV RF linac generating bremsstrahlung radiation permits detection of the variation of the high energy X-ray absorption or scattering factor across the container area and thus reconstructing an image of the container's contents. Currently, more than one hundred systems based on this technique are installed, mainly at seaports, worldwide and are used to detect contraband.
2. The second direction is based on more complicated processes, including nuclear processes—slow and fast neutron capture and scattering, high-energy monochromatic X-ray absorption, photonuclear reactions, and delayed neutron registration. Methods being developed do not aim to reconstruct details of the container content, but rather to produce an alarm signal if explosive or fissionable material is present in the container. Although early installations based on slow neutron capture were developed and installed at airports in 1980s, no commercial product currently is capable of operating with low levels of false alarms and high output. The main reasons for that are the low cross-section (probability) of the nuclear reactions, resulting in low levels of response signal; the absence of the probing particle sources with appropriate parameters; and the limited capabilities of the particle detectors.
In most cargo inspection systems operating over the world (except some Chinese systems), the installation is a system based on the first direction discussed above using a machine marketed as Lintron-M, made by Varian Medical Systems. This machine was initially developed for medicine and defectoscopy and has been widely used for many years. It is produced in variants with different fixed electron beam energies of 1.9, 3, 6, and 9 MeV. The size and weight parameters for the 9 MeV machine are:
HeightWidthLengthWeight(cm)(cm)(cm)(kg)Accelerating head6430142150Modulator1229276150RF source3461107136Cooling/thermoregulating51716275Control18483010
As can be seen from the first row of this table, the volume occupied by the Linatron-M accelerating head is 6.4 m 3.0 m 14.2 m=273 m3. The Linatron-M producing a 9 MeV beam requires about 5 MW klystron.
Recently, a development of the first direction has been proposed in which two different energy electron linacs, operating in alternation, would generate two end point energy bremsstrahlung X-ray radiation illuminating the same part of the container. The different dependence of the X-ray absorption or scattering cross-section on energy for different elements is the basis for recognition of the light or heavy elements content anomaly, e.g., nitrogen in explosives or plutonium in fissionable materials.